1. Field of the Invention
The present invention relates to a resonant pressure sensor. More specifically, the present invention relates to a resonant pressure sensor which is high in measurement accuracy, simple to fabricate, and low in cost.
Priority is claimed on Japanese Patent Application No. 2011-183857, filed Aug. 25, 2011, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
FIG. 13 is an explanatory diagram illustrating a main part of a resonant pressure sensor in accordance with the related art, which is disclosed, for example, in Japanese Unexamined Utility Model Application, First Publication No. H01-171337. Referring to FIG. 13, a silicon diaphragm 11 and a fixed portion 111 of the silicon diaphragm 11 are arranged in a predetermined fluid 112. In this case, silicone oil is used as the fluid 112.
In this case, each of resonant-type strain gauges 12 has an H shape. FIG. 13 illustrates a cross section of a portion in the vicinity of an H-shaped fixed end of the resonant-type strain gauges 12. A magnet 13 includes a yoke 131 and a permanent magnet 132. The yoke 131 functions as a vibration suppressor. The vibration suppressor (the yoke 131) is disposed such that at least one surface of the diaphragm 11 is close to one surface of the vibration suppressor (the yoke 131) so that the silicon diaphragm 11 does not resonate with the resonant-type strain gauge 12 due to density and viscosity of the fluid 112 between the diaphragm 11 and the vibration suppressor (the yoke 131).
A concave portion 113 is formed by the silicon diaphragm 11 and the fixed portion 111 of the silicon diaphragm.
A silicon substrate 40 forms a chamber 114 together with the concave portion 113. One surface of the silicon substrate 40 is fixed to one surface of the fixed portion 111 of the silicon diaphragm. A pressure-conducting hole 141 is provided in the silicon substrate 40 and conducts pressure into the chamber 114.
One surface of a pressure-conducting joint 50 is adjacent to and fixed to the silicon substrate 40 via a spacer 42 provided in an external opening portion of the pressure-conducting hole 141 so that the fixed portion 111 of the silicon diaphragm does not resonate with the resonant-type strain gauge 12 due to density and viscosity of the fluid 112 between the silicon substrate 40 and the pressure-conducting joint 50.
In this case, the spacer 42 is configured integrally with the silicon substrate 40. A through hole 51 is formed in the pressure-conducting joint 50, and is connected to the pressure-conducting hole 141 of the silicon substrate 40. A cover 60 is configured to cover the magnet 13, the diaphragm 11, the fixed portion 111, the substrate 40, and the spacer 42, and attached to the pressure-conducting joint 50. The inside of the cover 60 is filled with the fluid 112.
In the above-described configuration, when external pressure is applied to the diaphragm 11, the natural frequency of the resonant-type strain gauge 12 changes according to the external pressure. A vibration of the resonant-type strain gauge 12 is detected by a vibration detecting unit, and a detected frequency is extracted as an output signal.
As a result, the external pressure applied to the diaphragm 11 can be detected. Further, since the vibration suppressor 131 is provided, and one surface of the vibration suppressor 131 is provided to be adjacent to at least one surface of the diaphragm 11, the silicon diaphragm 11 does not resonate with the resonant-type strain gauge 12.
In other words, the diaphragm 11 has a resonant frequency depending on its shape but the resonant amplitude is suppressed by the silicone oil 112 in a gap between the diaphragm 11 and the yoke 131. Thus, even when an oscillating frequency of the resonant-type strain gauge 12 matches the resonant frequency of the diaphragm 11, the diaphragm 11 can be prevented from resonating. For example, in this example, this condition is sufficiently satisfied when a gap between the yoke 131 and the diaphragm 11 in silicone oil of 100 cs is smaller than 0.1 mm.
FIG. 14 illustrates an example in which a relation between Q and the gap a of the diaphragm 11 is actually measured in various kinds of fluids. It can be seen that when Q is smaller than 0.7, there is no substantial influence of resonance of the diaphragm 11. Here, A represents the case when the fluid 112 is air. B represents the case when the fluid 112 is Freon. C represents the case when the fluid 112 is silicone oil.
FIG. 15 is an explanatory diagram illustrating a main part of a resonant pressure sensor in accordance with the related art, which is disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. H02-032224. Referring to FIG. 15, a damping base 19 made of single crystalline silicon is received in a concave portion 11. A flow hole 20 is formed at the center. A bottom portion of the flow hole 20 bonds with a base chip 16 in a thermal oxidation manner to be connected to a through hole 15.
A predetermined gap Δ is maintained between an upper portion of the damping base 19 and a bottom portion of a diaphragm 13. Since silicone oil 21 is sealed inside the concave portion 11, influence of a resonant-type strain gauge 14 is damped by the narrow gap Δ and thus removed.
FIGS. 16 to 23 are explanatory diagrams illustrating a main part of a resonant pressure sensor in accordance with the related art, which is disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. H06-244438. First, as illustrated in FIG. 16, a spinel epitaxial layer 12 is formed on one surface side of a semiconductor substrate 11. Next, as illustrated in FIG. 17, an oxide silicon film 13 is formed between the semiconductor substrate 11 and the spinel epitaxial layer 12.
The spinel epitaxial layer is described in, for example, “SOT Structure Forming Technique,” p. 259, written by Seijiro Furukawa, issued from Sangyotosho on Oct. 23, 1987. The spinel epitaxial layer 12 is a film taking over the crystallographic structure from the semiconductor substrate 11. Next, as illustrated in FIG. 18, a poly-silicon layer 14 is formed on the surface of the spinel epitaxial layer 12, and the poly-silicon layer 14 is converted into a single crystalline layer by an annealing process.
Then, as illustrated in FIG. 19, portions of the poly-silicon layer 14, the spinel epitaxial layer 12, and the oxide silicon film 13 are removed through a photolithography technique and an etching technique such as a reactive ion etching (RIE) technique. A reference numeral 15 denotes a resist.
Next, as illustrated in FIG. 20, a silicon epitaxial growth layer 16 is formed on the surfaces of the semiconductor substrate 11 and the poly-silicon layer 14. Next, as illustrated in FIG. 21, a strain detecting sensor 17 is formed on the silicon epitaxial growth layer 16. In this case, a piezo-resistive element is formed.
Next, as illustrated in FIG. 22, the other surface of the semiconductor substrate 11 is etched up to the oxide silicon film 13 to form a conducting hole 18. Next, as illustrated in FIG. 23, the oxide silicon film 13 is removed by performing selective etching through the conducting hole 18. As a result, the diaphragm and a gap chamber are formed.
In short, in this kind of resonant pressure sensor, as pressure is applied to the diaphragm, the diaphragm is deformed. As the diaphragm is deformed, deformation occurs in the resonant-type strain gauge, and thus the resonant frequency of the resonant-type strain gauge changes. The pressure applied to the diaphragm can be measured by detecting the frequency change.
In the resonant pressure sensor using the resonant-type strain gauge, the resonant-type strain gauge is self-excited using an external circuit, and thus it is preferable that energy supplied from the circuit be used only for vibration of the resonant-type strain gauge.
However, part of energy input to the resonant-type strain gauge is expended as resonance energy of the diaphragm at a frequency at which the resonant frequency of the diaphragm is the same as the self-excitation frequency of the resonant-type strain gauge. As a result, since the Q value of the resonant-type strain gauge element is lowered, characteristics such as input/output characteristics worsen. As a method of solving this problem, a technique of suppressing resonance of a diaphragm using a narrow gap filled with oil has been proposed.
As concrete examples for implementing this technique, a method of arranging a machined part at a side at which the resonant-type strain gauge is arranged and forming a gap (Japanese Unexamined Utility Model Application, First Publication No. H01-171337), a method of arranging a convex portion in a part facing a concave portion of a diaphragm (Japanese Unexamined Patent Application, First Publication No. H02-032224), and a method of etching an oxide layer and forming a gap (Japanese Unexamined Patent Application, First Publication No. H06-244438) have been proposed.
As a manufacturing method in accordance with the related arts, particularly, as a method of forming a diaphragm, there are a method of forming a diaphragm using deep alkaline etching described in Japanese Unexamined Utility Model Application, First Publication No. H01-171337 and Japanese Unexamined Patent Application, First Publication No. H02-032224 and a method of forming a diaphragm using an oxide film described in Japanese Unexamined Patent Application, First Publication No. H06-244438 as an etching stopper.
FIG. 24 is an explanatory diagram illustrating a main part of a resonant pressure sensor in accordance with the related art. In the method of forming a diaphragm using deep alkaline etching, a single crystalline wafer 101 is subjected to anisotropic wet etching to form a diaphragm 102 to a desired thickness. The thickness is controlled based on an etching rate and an etching time. In this technique, as illustrated in FIG. 24, a concave portion configured with a surface (111) 103 is formed.
In the method of forming a diaphragm using an oxide film described in Japanese Unexamined Patent Application, First Publication No. H06-244438 as an etching stopper, etching by an alkaline solution and plasma etching are used, but since an oxide film can be used as an etching stopper, the film thickness can be controlled with a higher degree of accuracy than the method using deep alkaline etching.
However, the above-mentioned methods have the following problems. In the technique disclosed in Japanese Unexamined Utility Model Application, First Publication No. H01-171337, since a diaphragm is formed using deep alkaline etching, it is difficult to control the thickness of several μm to several tens of μm in units of μm, and thus it is difficult to suppress a variation in sensitivity.
Further, since a gap is formed using a machined part, it is difficult to form a small gap of several tens of μm or less with a high degree of accuracy, and there is a limitation to suppressing resonance of a diaphragm. Further, since a machined part is used, a foreign substance may be mixed in when a gap is formed. In this case, a movable range of a diaphragm is limited, and thus characteristics such as input/output characteristics may be affected. In addition, it is difficult to freely select the shape of a diaphragm due to influence of crystal surface orientation by alkaline etching when a diaphragm is formed. For this reason, there is a design restriction to the shape of a diaphragm, and it is difficult to design the shape of a diaphragm with flexibility.
In the technique disclosed in Japanese Unexamined Patent Application, First Publication No. H02-032224, since a diaphragm is formed using deep alkaline etching, it is difficult to control the thickness of several μm to several tens of μm in unit of μm. Further, when a gap is formed, it is difficult to process a concave portion by alkaline etching with a high degree of accuracy, and a processing error of a facing convex part lowers the accuracy. Thus, it is difficult to form a gap of several μm with a high degree of accuracy, and there is a limitation to suppressing resonance of a diaphragm.
Further, it is difficult to freely select the shape of a diaphragm due to influence of crystal surface orientation by alkaline etching when a diaphragm is formed. Thus, when a diaphragm is designed, there is a restriction to the shape of a diaphragm, and it is difficult to design a diaphragm with a flexible shape.
In the technique disclosed in Japanese Unexamined Patent Application, First Publication No. H06-244438, since an oxide film is used to form a gap, stress occurs in the boundary between an oxide film and silicon, and a wafer may be bent or an oxide film cracked. The limit of the oxide film thickness to avoid this state is about 3 μm to about 4 μm, and thus it is difficult to form a gap of about 3 μm to about 4 μm or more.
For this reason, a movable range of a diaphragm is restricted by a gap, and there is a restriction on a design of a sensor with a pressure range in which displacement of 4 μm or more is necessary. Further, a diaphragm is formed by epitaxial growth and is affected by a crystal surface. Thus, in order to form a diaphragm having high breaking stress with no crystal defect in a boundary portion between a substrate and a diaphragm, a design needs to be made in consideration of a crystal surface. For this reason, it is difficult to design a diaphragm with a flexible shape which is not restricted by crystal orientation.
Next, among the fabricating methods in accordance with the related arts, particularly, a problem of a method of forming a diaphragm will be described. The deep alkaline etching described in Japanese Unexamined Utility Model Application, First Publication No. H01-171337 and Japanese Unexamined Patent Application, First Publication No. H02-032224 have the following problems. First, the deep alkaline etching is easily affected by the temperature of a chemical, and thus it is difficult to control the thickness. Further, even though an etching amount is large, the accuracy required for a film thickness of a diaphragm is high, and thus it is difficult to control. In addition, at the time of etching, it is necessary to protect an element surface from a chemical.
Meanwhile, in the method using an oxide film as an etching stopper, which is described in Japanese Unexamined Patent Application, First Publication No. H06-244438, a gap is formed depending on the thickness of the oxide film. For this reason, it is difficult to form a gap of about 3 μm to 4 μm or more that causes a wafer to be bent or an oxide film to be cracked. In this case, the diaphragm comes into contact with an opposite structure, and a movable range of the diaphragm is restricted. In other words, there is large restriction on the design of a sensor with a pressure range in which displacement of 4 μm or more is necessary.
FIG. 25 is an explanatory diagram illustrating a main part of a resonant pressure sensor in accordance with the related art. In the deep alkaline etching, as illustrated in FIG. 25, when a diaphragm forming process is performed at the same diaphragm thickness with the mask of the same shape, diaphragms having different finished shapes are formed in wafers that differ in the thickness due to the difference in an etching speed in the plane orientation.
For this reason, wafers that differ in inch size have different thicknesses, and thus it is necessary to change an etching condition or a mask pattern. This means that after a prototype wafer having a small inch size such as 4 inches is fabricated in a research and development phase, in order to commercialize and mass-produce a wafer having a large inch size such as 8 inches or 12 inches, it is necessary to make a mask again and change a manufacturing condition. Thus, in order to migrate from trail production to commercialization, an extraordinary amount of time is inevitably expended.
In addition, in the pressure sensor, the shape and the thickness of a diaphragm need to be designed again according to a pressure range. In order to implement diaphragms of diverse thicknesses corresponding to various pressure ranges by deep alkaline etching, a mask pattern and a fabricating condition need to be individually managed and manufactured for each thickness of a diaphragm.
Further, even the diaphragm fabricated using the epitaxial growth discussed in Japanese Unexamined Patent Application, First Publication No. H06-244438 is affected by the plane orientation, and thus it is difficult to design the flexible shape which is not restricted by crystal orientation.